1. Field of the Invention
The present invention relates generally to nuclear fuel pellet diameter inspection, and more particularly, to an apparatus and method for laser scanning of pellets to measure pellet diameter in a pellet production line.
2. Description of the Prior Art
A nuclear fuel pellet having an oversize or undersize diameter can greatly affect the efficiency and cost of manufacturing nuclear fuel rods. The oversize diameter pellet may clog the entry to the fuel rod, requiring the operator to turn off the pellet feeding apparatus, pull out the oversize pellet, and/or replace the affected fuel rod. This corrective measure wastes considerable manufacturing time.
Additionally, an oversize fuel pellet can cause premature and costly replacement of a nuclear fuel rod in the reactor core at a nuclear power plant. If a pellet is too large, the pellet-to-clad gap is reduced, leading to excessive clad stresses and potential fuel rod failure during core operation. On the other hand, undersize pellets reduce fuel rod efficiency due to lack of optimum amount of fuel within the rod.
In the past, diameters of pellets have been checked at random at two separate stages of the manufacturing process. First, one pellet per tray is inspected for diameter oversize/undersize with a hand micrometer by quality control personnel, and the official results are recorded. Second, the pellets are inspected at random with a hand operated micrometer by an operator as the pellets exit the grinding station. The tolerance for pellet diameter is plus or minus 0.0005 inch from nominal. Such measurements are noted but not recorded by the operators; they are merely used to ensure proper grinder adjustment.
More recently, a desire has grown to inspect 100% of the pellets produced, instead of conducting random tests. One inspection system presently being used employs a pellet transfer apparatus in conjunction with a laser scanning apparatus for scanning pellets with a laser beam to measure and detect oversize and undersize ones as they are moved, arranged end-to-end in a single file row, from the grinding station to a pellet storage tray. However, this system has several drawbacks which prevent it from achieving satisfactory inspection results.
One drawback is that to move the long pellet row the pellet transfer apparatus has to push the row by gripping the rearmost pellet in the row. Pushing from the rear of the pellet row typically produces upward buckling of some of the pellets at the middle of the row. Such buckling results in inaccurate diameter measurements and a false indication of oversize pellets.
Another drawback is that the pellet transfer apparatus employs a rotating flywheel connected peripherally by a link to a reciprocable carriage to drive the carriage and a pellet gripping mechanism mounted thereon through drive and return strokes. The constant rotary motion of the flywheel is converted into a reciprocable motion of the carriage having a non-uniform or non-constant velocity. Pellets in the middle of the pellet row move past the laser at a higher speed than at the opposite ends such that the laser may only achieve one measurement for each middle pellet.
Thus, not all pellets move past the measuring laser at the same speed which diminishes the accuracy in and number of readings taken. Since it only takes a localized oversize diameter condition on one of the pellets to cause problems in its subsequent insertion into the fuel rod and in increasing clad stresses, a decrease in the number of readings is just the opposite of what is desired.
A further drawback is that the beam of the laser is aligned perpendicular to the axes and the direction of travel of the pellets. Such orientation permits the chamfers on the opposite ends of adjacent pellets to show up as a reduced or undersize pellet diameter thereby giving false readings. Chamfers may reduce a nominal diameter reading by as much as 40 mils if the scanner emits a laser beam perpendicular to the central axis of the pellet.
Consequently, a need exists for a different way to transfer pellets and measure pellet diameters so as to avoid the above-mentioned drawbacks.